1. Field of the Invention
This invention pertains generally to supercapacitors, and more particularly to supercapacitors with nanowire composites.
2. Description of Related Art
An ideal electrical energy storage device provides both high energy and power density. The state-of-art electrical energy storage technology is primarily based on either lithium ion batteries with low power density (normally less than 1000 W/kg) or electrical double layer capacitors (in the range of 3-6 Wh/kg), which limits their wide application in heavy-duty facilities, electric vehicles, mobile electrical tools and consumer electronics.
Supercapacitors are emerging as a new-generation of energy storage device. However, with aqueous electrolyte based supercapacitors, which are limited by low cell potential, it is difficult to achieve high energy density. Current supercapacitors utilizing active carbon in organic or ionic liquid electrolyte have energy densities approaching 10 Wh/kg. While supercapacitors exhibit significantly higher power densities compared to batteries, but their use in electronic devices and industrial applications is limited based on their poor energy density.
Since the energy density (E) of a capacitor is governed by E=½CV2, where C is the capacitance and V is the cell potential, increasing the potential or capacitance leads to higher energy density. In this context, the most commonly used electrode material (porous carbon) generally possesses double layer capacitances of around 100 F g−1, which can provide a specific energy density up to 25 Wh kg−1 in an organic-electrolyte based symmetric device. Somewhat greater energy densities can be reached as specific capacitances of up to 150 Fg-1 with carbide-derived carbon have been reported. By comparison, transition-metal oxides possess significantly higher specific capacitance via pseudocapacitance. For example, RuO2, MnO2 and NiO have demonstrated specific capacitance up to 1300, 1200, and 940 F g−1, respectively. Some devices are based on the TiO2 (B) nanowire or Li4Ti5O12 as anode and activated carbon (or CNTs) as cathode, which provide devices with an energy density from 10-20 Wh/kg.
Thus, an asymmetric supercapacitor consisting of a carbon cathode and an oxide anode may provide a significantly higher energy density than symmetric capacitors based on carbon; asymmetric cells containing an anode of Li4Ti5O12 and a cathode of activated carbon (AC) can provide an energy density in excess of 35 Wh kg−1. Nevertheless, building such high-energy density asymmetric devices has been highly challenging, mainly due to the kinetics of the pseudocapacitive electrode. In order to minimize the kinetic limitations, most of the pseudocapacitive electrodes made today are limited to sub-micron thin films. Since the electrochemically inert components of a supercapacitor, including the current collectors, separator, and packaging, account for a large fraction of the total weight of the device, the use of thin electrodes results in a significantly lower energy density than what could be attained using thicker electrodes.
Accordingly an object of the present invention is energy storage system that includes both high power and high energy density.
Another object is an asymmetric supercapacitor coupling both a Faradaic electrode and double-layer electrode in a single cell to get higher energy density as well as high power density and cycling life.
Another object is a thick electrode for supercapacitors for making high-energy supercapacitors for practical applications.